In a conventional type of servo control system having a host computer and a plurality of servo controllers connected to the host computer, the host computer and each of the servo controllers are connected to each other through a serial communication line such as RS-232C so that data transaction is executed asynchronously to each other.
FIG. 24 is a block diagram showing a NC system disclosed in Japanese Patent Laid Open Publication No. HEI 1-195509. A plurality of NC units are connected to a unit of host computer through RS-232C in this NC system as shown in the figure. In this example of the NC system, the host computer 163 has a plurality of RS-232C interfaces, and interface conversion boards are connected to the plurality of RS-232C interfaces respectively. Each of the interface conversion boards is connected to each of the NC units through a tape reader interface DLM, and a plurality of NC units are connected to the host computer 163 through the plurality of interface conversion boards 160-1 to 160-n.
Further connected to this host computer 163 is an NC unit with a remote buffer through another RS-232C interface.
This NC system is possible to execute processing, from the host computer 163 to NC units 170-1 to 170-n, such as setting or changing of parameters, write/read of a control program, and intermittent monitoring of internal data in the NC units 170-1 to 170-n.
In the servo system with the host computer and servo controllers connected to each other through serial communication lines such as RS-232C each for executing data transaction asynchronous to each other, a communication speed was slow and communications were executed in asynchronism to a control cycle of each of the servo controllers, so that, assuming a case as an example where internal data in any of the servo controllers was monitored from the host computer, dynamic changes in the internal data in the servo controller could not be monitored with high precision because monitoring could be executed only in a comparatively longer cycle as compared to the control cycle for the servo controller and also a sampling time was not accurately specified.
For this reason, a speed and a current value or the like of a servo motor had to be monitored by using a large-sized and expensive measuring device such as a digital storage oscilloscope for adjustment required at the time of getting the servo system up and running or the like Also, the above circumstances required works near a high voltage and a large current circuit such as a distribution panel, which has raised concerns for safety.
If processing, like transfer of a large scale file, requiring occupation of a communication line for a long period of time is executed, processing such as monitoring can not be executed during this period of time.
When a case such as welding of various works each having a complicated form or coating thereof needs a large amount of point data and also needs to rewrite a machining program quite often, point data and a machining program are frequently transferred from the host computer to the servo controllers, and for this reason some troubles occur in monitoring of a current position or the like of the servo motor by the host computer or in transmission of an instruction for start from the host computer to the servo controllers.
When any unit with comparatively large fluctuation in load is to be controlled, there is employed a method of correcting an instruction itself so that target movement coincides with actual movement. When this method is employed in a line for conveyor such as a belt conveyor device in which changes in load are comparatively frequent and for which a plurality of servo controllers are used, high-capacity memories each with software for realizing the function stored therein for execution have to be provided in the servo controllers respectively. In addition, if this method is employed, a load to the CPU increases, and for this reason, functions to be executed originally by the servo controller such as processing for selecting and using any of a plurality of algorithms corresponding to each object to be controlled or processing for replacing algorithms with each other are prevented.
When data is to be written from the host computer to the plurality of servo controllers, processing for writing data to each of the servo controllers by matching control cycles of the servo controllers to each other respectively and synchronizing among the servo controllers can not be executed.
There has been provided, in the conventional type of system, an application program operating on a general-purpose OS such as MS-DOS.RTM. or Windows.RTM. from vendors of servo controllers so that users can set and change parameters in the servo controller for the system or write and read a control program to and from the servo controllers, but it is difficult to prepare and supply an application program so as to enable construction of a man-machine interface matched to an actual device (e.g., various types of man-machine interface each matched to a user's system in which a picture simulating an actual device such as a line for conveyor is displayed on a screen of the host computer and monitor sections such as switches are displayed in the picture respectively) because access to an internal memory of any of the servo controllers according to an instruction from the host computer is not easily and quickly performed, which causes inconvenience in operability for a user.
When a large amount of point data to be set and a machining program to be rewritten frequently cause a communication line between the host computer and the servo controllers to be occupied, processing such as monitoring can not be executed with quick response. For example, when the host computer always monitors a status of each servo controller and needs to execute appropriate recovery processing when an error occurs, this recovery processing is not executed quickly, which is disadvantageous.
It is also hard to operate a plurality of servo controllers synchronously to each other.
Although there has been employed a method of electrically synchronizing steps to each other by controlling a large number of servo motors by one unit of servo controller, this method has some problems in the point that a longer cable is required as the number of axes to be controlled increases, and in addition, it is difficult to divide a program for each step for management due to increase of load on the servo controller, and it is also difficult to prepare the program.
A machine such as a large-sized printing machine for machining in a plurality of steps while paper wound into a roll is fed out requires synchronization between a timing for feeding out the paper and a machining operation in each step, so that, in the conventional type of system, synchronization is realized by using a main shaft which is a reference as a driving source and also using cams and gears or the like rotatably driven by this main shaft, but this method requires a large-sized and expensive device, and also requires maintenance of the machining device.
The conventional type of system disadvantageously requires particular software on the host computer for the purpose that the host computer sets and changes parameters in the servo controllers or writes and reads a control program.
When any servo controller is to write data in other servo controller or the servo controller is to read out data from other servo controller, data transaction is conventionally executed therebetween through a programmable controller, but a sequential program for realizing the operation should be prepared by a user, which may need a long time for preparing the program.